The present invention relates to a method and an apparatus for evaluating the layer thickness of a photoreceptor having a plural number of layers layered on a conductive substrate formed thereon. The invention relates to a method and an apparatus for manufacturing photoreceptors.
In the electrophotographic apparatus, such as copying machines and printers, the photoreceptor used therein having a layered structure consisting of an under coat layer, a charge generating layer, and a charge transport layer successively layered on a conductive substrate is known. In manufacturing the photoreceptor, photosensitive coating liquid is first prepared by dissolving or dispersing organic photoconductive material and binder resin into organic solvent, a conductive substrate is successively coated with the coating liquid, and then the resultant is dried.
Various methods of coating the photosensitive coating liquid, employed in the photoreceptor manufacturing process, are known. Of those known coating methods, a dipping method is widely used because of high productivity. In the dipping method, a conductive substrate is dipped into a bath containing coating liquid, and pulled out of the bath at a preset speed.
However, this dipping method is disadvantageous in that the photoreceptor is likely to be formed droopy. This brings about various drawbacks, for example, an irregular coating of the photosensitive layer, formation of stripes, great thickness difference between the upper part of the photosensitive layer and the lower part, and the like. This leads to formation of a defective image, for example, an image having an irregular optical image density. Organic solvent, easy to volatilize, is frequently used for the coating liquid. Where such a solvent is used, the solvent volatilizes from the coating liquid in the bath, and the viscosity and concentration of the coating liquid varies. Consequently, it is difficult to perform the coating process under fixed conditions.
In manufacturing the photoreceptors, to detect some variation in the coating process, the thickness values of the formed layers are measured and evaluated. The results of the evaluation are fed back to the coating process to adjust the thickness. The film thickness measuring methods, currently used, are generally categorized into a contact film-thickness measuring method and a noncontact film-thickness measuring method. The former method uses a step measuring meter, an eddy-current film-thickness measuring meter, or the like. The latter is based on a color difference method, an interference method, a light absorption method, or the like.
The film-thickness evaluation by the interference method utilizes the interference effects by a multiple reflection. It is frequently used for measuring the thickness of the transparent layers, for example, the under coat layer and the charge transport layer because of its easy and short time evaluation. A technique disclosed in Japanese Patent Laid-Open Publication No. Hei. 4-336540 uses this film-thickness evaluation to measure and evaluate the thickness of the under coat layer and the charge transport layer, and controls the coating rate in the coating process in accordance with the evaluation results.
The film-thickness evaluation by the light absorption method utilizes such a nature that a quantity of light absorbed by the film varies with the film thickness. It is frequently used for measuring the thickness of the layers of the photoreceptor because of its easy and short time evaluation. In this method, infrared absorption is used for measuring the thickness of the transparent layers, for example, the under coat layer and the charge transport layer. For measuring the thickness of the layer containing pigment dispersed therein, such as the charge generating layer, visible light absorption is frequently used.
Practical characteristic values obtained by the light absorption method are typically the absorbance and the reflectivity. These characteristic values are each expressed as a ratio of the quantities of reflecting light before and after the film formation or a ratio of the quantities of light incident on and reflecting from the film formed. The characteristic values often correlates with the thickness of a film to be measured. In this case, the characteristic value may be converted into the corresponding film thickness by using a conversion formula previously formed. A method for converting the characteristic value to the film thickness is disclosed in Japanese Patent Laid-Open Publication No. Hei. 6-130683. In this method, a partial area having no under coat layer formed therein is formed outside the image forming area on the surface of the conductive substrate, and a charge generating layer is formed on this area. A spectrophotometer gathers light absorbed in the partial area and produces a spectrum of the light. A ratio of the quantities of light of a specific single wavelength incident on and reflecting from the partial area, viz., absorbance, is calculated from the spectrum, and converted into the corresponding thickness of the film.
In calculating the absorbance from the ratio of the quantities of light incident on and reflecting from the coated film by using the spectrophotometer, for example, the reflecting light is measured by the spectrophotometer, and hence the quantity of light obtained is only the quantity of the reflecting light. Therefore, it is necessary to use a sensor for measuring the quantity of the incident light indispensable for the calculating of the absorbance. In the method using the sensor, it is estimated that optical path variations in the optical system, viz., a variation of the quantity of light from a light source and a variation of the distance between an object to be measured and the light source, greatly affect a variation of measured values. For this reason, this method is not practical. When the reflectivity of the conductive substrate per se varies, that is, the reflectivity values of the objects to be measured are not uniform, the quantity of the reflecting light irrelevant to the light absorption by the film varies. The result is a great variation of the measured values. The film thickness of the photoreceptor is not always uniform. This necessitates the irregularity evaluation, such as thickness irregularity in the circumferential and axial directions, and droopy state of the film.
The area on the photoreceptor that can be evaluated in its thickness by the converting method as described in Japanese Patent Laid-Open Publication No. Hei. 6-130683 is only the partial area having no under coat layer, viz., the upper end of the film. In case that the method of the publication is applied to an area having the under coat layer formed thereon, if the adverse effect by the interference is canceled, a variation of the thickness of the under coat layer leads to a variation of the optical path, and hence a great variation of the measured values. In this respect, this method is also not practical. It is known that the film at that location has a great thickness irregularity. Therefore, the irregularity evaluation, such as thickness irregularity in the circumferential and axial directions, and droopy state of the film, is essential. In this sense, the thickness evaluation method under discussion needs some improvement.
The measurement of the thickness of the layers of the photoreceptor may be applied to the evaluation of the characteristic value of the photoreceptor, which has a correlation to the thickness of the photoreceptor layers. For example, an electrical characteristic correlating with the thickness of the charge generating layer may be evaluated on the basis of the thickness of the charge generating layer. An evaluation method for evaluating a sensitivity to light having a relation with the thickness of the charge generating layer is disclosed in Japanese Patent Laid-Open Publication No. Hei. 4-52653. These characteristic values per se are often used as substitute characteristics to control the coating process.
In some photoreceptors which require a process for preventing interference fringes, for example, those used for digital color copying machines, printers, and the like, the roughness on the substrate surface and the interfaces of the under coat layer are set at high values. Various methods to make those surfaces rough are known and practically used. A first category of the surface roughening methods mechanically roughens the surfaces, and includes a Horning method, an etching method, a steel ball drop/impact method, a roughened cylinder applying method, a grinding/cutting method, a laser irradiation method, a high-pressure water jetting method, and the like. A second category of the roughening method chemically processes or oxidizes the substrate surface, and includes an anode oxidizing method, a bermite process, a heat oxidizing method and the like. A third category of the roughening method places an intermediate layer for preventing interference between the photosensitive layer and the surface of the substrate.
The roughness values of the surface roughness of the thus processed conductive substrate are not uniform over the same surface and among different conductive substrates. In the evaluation method based on the light absorption, the light reflected from the layer of the photoreceptor is affected by the surface roughness, so that the quantity of the reflected light varies. Accordingly, the varied roughness values bring about a large variation of the characteristic values based on the quantity of the reflecting light, such as the absorbance and the reflectivity. This results in poor measurement accuracy. When the characteristic values of the film thickness, i.e., the absorbance and the reflectivity, are used as substitute characteristic, these values are also greatly varied. Further, when the coating process is controlled by using the characteristic values, the evaluated values greatly deviated from the true values will be fed back to the coating process.
To remove the variation of the roughness values among different substrates, the absorbance and the reflectivity may be obtained from the quantities of light absorbed by the layer and the light reflected from the same before and after the coating process. To realize this, the same measuring conditions must be set for both the measurements before and after the coating process. However, it is difficult to set up the same conditions since if the same measuring equipment is used, different coating processes are carried out at different times. Accordingly, the power source voltage varies, and with the variation of the power source voltage, the quantity of light from a light source varies, and hence the characteristic values, such as the absorptive value and the reflectivity, also vary.